Different species have very different essential nutrients. Most essential nutrients are substances that are metabolically necessary but cannot be synthesized by the organism. Dietary minerals, for example, cannot be synthesized in biological systems, so (for example) a human must obtain the iron they need to build hemoglobin from their diet. Of course, this iron is recycled, but some is inevitably lost, for example during menstruation.

All essential nutrients are toxic in large doses (see hypervitaminosis or the nutrient pages themselves below). Some can be taken in amounts larger than required in a typical diet, with no apparent ill effects. Linus Pauling said of vitamin B3, (either niacin or niacinamide), "What astonished me was the very low toxicity of a substance that has such very great physiological power. A little pinch, 5 mg, every day, is enough to keep a person from dying of pellagra, but it is so lacking in toxicity that ten thousand times as much can [sometimes] be taken without harm."
[1] A similar statement can be made about vitamin C and some other vitamins.